Wave guide antenna system



Allg- 12, 1952 w. E. KocK 2,607,010

wAvE GUIDE ANTENNA SYSTEM A TTORNEV Aug. 12, 1952 w. E. KocK 2,607,010

WAVE GUIDE ANTENNA SYSTEM Filed April 23, 1945 3 Sheets-Sheet 3 ATTORNEV 3 sultants have opposite phase angles at the primary antennaaperture. Hence the feedback components from the small and largereflectors mutually cancel at the antenna aperture and the formation ofstanding waves is avoided in the guide. In more detail, the aforesaiddimension a of the small reflector is mathematically related to the meanwavelength of the system, the focal length of the large reflector, andthe-.parameter of the cornu spiral representing the integratedamplitudes and phases of the components in the circular wave frontestablished by the primary antenna aperture. The separation of thereflective faces of the two reflectors is such as to produce mutualcancellation of the wavelets returned to the primary antenna by thelarge reflector and the auxiliary reflector.

In accordance with a second embodiment, a

paraboloidal reflector and a disk or plate auxiliary Y acylindrical-parabolic. reflectorl equipped with parallelend plates. Inaccordance with a fourth embodiment, the main reflector hasacylindrical- `paralmlic contour and a double strip auxiliary reector is4atta'czhed tothe vertex of fthe main reflector;V and-'inV accordancewith'a fifth embodiment the antenna system comprises a main paraboloidalreflector and an annular or`ring reflector. `The invention will be morefully understood from. a perusal of the following specification takeninconjunction with the drawing on which like 1 reference. charactersdenote elements of simllar'function and on which:

` Fig. l is a sectional front view of a ilrst embodimentof vtheinvention comprising a cylindricalparabolicreflector anda singleauxiliary linear strip reflector; 'l

Fig. 2 is a sectional front view of a se'cond'embodiment'comprising aparaboloidal reflector and anauxiliarygcircular plate or disk reflector;,Y Fig. 3 isfa sectional side view of the'abovementionedfrst' and,second embodiments;r H

Fig. 4 is aperspective view of a third embodiment comprising acylindrical-parabolic reflector, end plates therefor and a single-stripreflector;

Figs. 5 and 6 are explanatory .diagramsused in explainingthe invention;i

Fig. 7 is a, cornu spiral and vector diagram used in determining thesize of the auxiliary reilectors =included in the embodiments of Figs.1, 2'an'd '4;'1 i... S Y

8 illustrates the measured bandwidth curves'for the embodiment of Fig. 4and a prior art systenrwithout the auxiliary reflector;

p Fig. 91s a sectional front view of a fourth embodiment comprising acylindrical-parabolic reflector and a pair of strip reflectors;

Fig. 10 is a sectional front view of a fifth embodiment comprising aparaboloidal reflector and an auxiliary lring reflector; Fig. l1 isasectional side view of the fourth and fifth embodiments, and

Fig, 12 is a cornu spiral and 'vector diagram used in determining thedimensions of'theauxiliary reflectors included in the embodiments ofFigs.9 and l0. v l

Referring lto Figs. 1 and 3, reference numeral I denotes ya largecylindrical-parabolic reflector having a vertex line 2, a focal linej3,a focal length R, a longitudinal dimension L1 and an axis or axial plane4. Numeral 5 designates a dielectric guide comprising the metallic walls6 and the air dielectric 1. The guide 5 is connected at one end to thetransceiver 8 and has at its other end an antenna aperture 9. Themid-point or center line of the antenna aperture 9 is on the focal line3 of reflector IV and faces the reflector. Numeral IB denotes a linearstrip reflector attached toreilector I and havingv a length L, athickness d5, a dimension or width 2a in the plane of the paraboliccontour of reflector' I, and a centerline or longitudinal axis I Iincluded in the axial plane 4.

In operation, microwaves of the given or design frequency are suppliedby the transceiver 8 and conveyed over guide 5 to the antenna aperture9, and thence emitted towards the main and auxiliary reflectors I andIU. The arrows I2 and I3 represent typical directions for the waveletsradiated by aperture 9. The front of the emitted wavelets, in the planeof the parabolic contour of reilector. l, vis substantially circular.`The wavelet's impinging upon reflectors` If and I8 are reflected orre'radiated, as Well understood in the art, in diverse directionsillustrated. by way of example, byarr'ows I4, I5, I5, I'I'and I9; andmaximum radio action occurs in direction I9 ineluded inthe axial plane4. The main beam of the antenna system 9, I is relatively wide in theaxial plane 4 and relatively narrow in the plane of the paraboliccontour, or latus rectum plane, so that the system constitutes afan-beam antenna. Asjshown by arrows I6A and I8 someof thewaveletsvreflected by the large reflector I 'and by the .small reflector I0 arereturned to the aperture 9 and guide 5. Assuming for the moment thatthestripv reflector IB is omitted, thel wavelets fed back by reflector Ivhave in guide 5 a propagation direction opposite to that of the'waves'supplied by the transmitter in device 8 and, as [a result,pronounced standing waves are established in guide 5. Stateddifferently, theguide 5, or more accurately the aperture 9, ismismatched or miscoupled to its load reflector I', wherebydetrim'ental`rellection occurs; lIn accordance with the invention, thestrip'reilectorl I0 isjutilized and, afs will now be explained,'its'thicknessd and width2uare` such that the'energies fedv back'by whilepermitting unilateral, coupling, of the active or primary antennaelement 9 and the passive or secondary antenna element I.

The embodiment of Figs. 2 and Bis structurally the same as that of Figs.1 and '3 except that 1a main paraboloidal reflector 2U and anauxiliarydisk reflector 2l are employed in place ofthe. cylindricalparabolic'reflector I and the strip. reflector I0. Also, the main beamof the antenna' system 9, 20 is relatively `narrow` in all planeslcontaining thev axis y22 of reflector 20 so that 'the system constitutesa point-beam antenna. The reflector 2D and the disk 2| are coaxiallydisposed. The thickness ds and diameter 2o'. ofthe" disk reflector 2|are the same as` the -thickness and the width of the strip reflector IU.In operation, the disk reflector functions to cancel`thevenergyreturnedby the main reflector 20 tothe primary antenna 4aperture9, whereby standing Waves fare eliminated in guide 5.1 Y

The' embodiment of Fig. comprises va cylindrical-parabolic reflector Iand a single-strip aux'f iliaryf-reiiecto'r I0 and-therefore?resemblesthe embodiment 'of Figs. 1 and-3 invcertainfvrespectS- The longitudinaldimension Lof the reflector'f/ Iv inthe lsystem of Fig. 4 is very shortas com-pared tol that of reflector I in Figs. 1,3; andin Fig-4theguidef5 extends perpendiculanto thelatus rectum of reflector Iwhereas in the embodiment ofiFigs."1,` 3 it-is parallel tothe latusrectum. Also, in the system yof Figil, the reflector lI is equipped witha conductive top end plate 23 and a'` conductive bottom endl plate'V '24which, to,-v

-gether withthe transverse ends of reflector-I,

form a rectangular'or substantially linear antennajapertur'e. Thesecondary antenna *comprising the cylindricaleparabolicreiiector Iv andthe end plates 23, 241constit`ute a so-calle'd pillbox, cheesefboxv vorlparallel plate"j antenna reflector. A conventional pill-box antennareflector is illustrated onl page 840 of the textbook Radio EngineersHandbook by F. E. Terman.

In general, the operation ofthe embodiment of' Fig. 4 is the same asdescribed above `for the cylirnirical-parabolic system of Figs. 1 and 3,and a fan-beam is obtained. The plates 23, 24 function as guide membersfor the energy lemanating from the primary antenna aperture 5 s o thatthe illumination or energization of reflector I is relatively intenseand a high gain is secured.'` The parallel plates also undesirablyin'-crease, at least to some extent, the feedback, or reflection fromreflector I towardsgthe primary antenna aperture 9. Hencatheutilizationof a strip reector I0, for the purpose of Vcancelling the'feedbackenergy, is'in a sense of more-significanoe-in the case of the system o'fFig'4 thaniin the system of Figs. 1, 3.

Referring to Figs. 5, 6 and 7 the method of determining the dimensions dand 2a of the strip and disk reiiectors I0, 2I will now be explained.VFirst of all, the amplitude of vibration 'at a: point P, Fig. 5, of thewaves received from a source S via a reiiector 25, the point P andsource S "being at thejrespective distances Rp and Rs from the surface'25, may be represented bythe upper'half 26 of a cornu spiral 21, Fig.7, as explained in. the textbook Theoretical Physics by Georg Joos,pages 376-378. Thus,`if the width or height of surface 25 were infinite,the'amplitude at point P may be represented by the vector 28, Fig. 7;extending from the origin 29 to. the asymptotic point 30, that is,between the opposite terminals, of the half spiral 2S. The lengthdimension of vector 23 represents the resultant yor integrated amplitudeof the wavelets originating ati'Sand impinging on the surface25;V andthe angle between the vector 28 and thev XX axis represents theintegrated phase of'these wave-y let`s.-`` If'v the width a ofthesurface 25 is` finitev4 andl has a dimension corresponding toarr-intermediate point on thespiral A, that is, correspondin'gfto aselected value of the'paramete'r-A*I ofthe spiral 26, an amplitude isobtained at thev 6 :Applying the above theory-to the embodiments ofFigs. 1, 2 fand 4, 'the.source'Sand pointl P vcor-l respond to theaperture 9 so itlia't RPT-R, vFig.l3, thefocal length of reflector FI,Figs. -1land4 or reflector 25,"`-Fig: 2. VAlso the parabolic curva--reflectors reasonably approximate the at surreceiving point P which maybe represented byf the `vector 3| extending from ther origin 23v to the.intermediate point. If thesurface 25k be'- gins at an intermediatepoint, Fig.-7,-correspondr face 25, Fig. 5. It is apparent from theabove ,that the Sulff?. 25, Eig. 5, maybe dividedinto two parts 33 and34, Fig. 6, which have somewhat critical widths and which produce twovectors 3| and 32 having equal lengths and therefore equal amplitudes.zMoreover,1 the thickness d or dierence in the lengths of the pathsextending between .the source S or point P and the two parts 33 and 34may be selected so that vector 3| assumes "apsition devoted by"v numeral35 and extending'in a direction .oppositetothat of vector 32, wherebythe integrated amplitude of vibrationat ,pointP "of the wavesVreceived.- from Sis zero., ."1 M 'A f 0VThe.relation*between thewavelength Xgt'he width@ ofthe Vreflecting member 33,'t`he dis"-tancesRP and Rs betweenthe surface 34ancll the V'point P. and source- S,4is Vgivenas equation (.35) on page 377 ofthe Joos textbook., andiswhere-A is-.the mean or design 'wavelength and,`

It follows Afrom the above that the critical or; optimum dimension2a`for1thewidth of the natv strip reector IILzFigs. 1, 3 and 4, used inthe embodiments comprising. a cylindrical .parabolic reflector having a.focal Ylength -of 30.5 inches,

is5.24f or 1.33wavelengths. VFor thecomparable paraboloidal system,Figs. 2 and 3, theoptimum diameter 2a. of the.' nat Adisk refIectorfZ-Iis,5.2ir inches. Ifthe renecting'f.surface-.of the'stiip orf'disk`reflector is parabolic,insteadof flat, the. parameter A is slightlylarger than.0.47. as, for.

example, 0.6 andthe dimension 2a greater-,than 5.24 inches.

The thickness d ofthe strip reflector I0, andk ofthe disk reflector 2|,may be determined from Fig. 7 by scalingthe phase angles .031 and 032,

is. vslightly,

respectively, or vectorsjSI and 32 and rotating vector- 3I` through anangle 0 to the position illustrated'by vector 35 at, which the sum ofthe phase` angle 035 and Athe `phase. angle osais 180 degrees.- Thethicknessd in wavelengths lof each of reectors I and A2 I corresponds toonehalf ofwthe phase angler. As scaled from Fig. 7,. equals about 1013degrees.A In the tested embodiment of Figs. 1 and 3, the thickness l 03degrees d T by reason of the front position of the primary antennaelement. The primary element shadows the 'vertex portion of the mainreector and the attached auxiliary reflector, and the auxiliary reectordoes not, therefore, appreciably affect either pattern. The mutual wavecancellation, however, obtained in transmission at the primary antennaand described above, diiers considerably from that obtained inreception. The cancellation of the wavelets reflected by ther main andauxiliary reflectors into the primary antenna or aperture 9,.. Fig. 3,is predicated on the assumption or condition that the outgoing waveletstravel along divergent radial paths of diierent length from the sourceS, Fig. 6, or aperture v9, and the components of the reected wavesreturn to the aperture 9 travelalong con-.V vergeht radial paths ofdifferent lengths. The cornu spiral, Fig. 7, and theassociated equations(l) and (6) from which the value of dimension c of the auxiliaryreiiector is obtained, are based on the assumption or condition justdescribed. This condition obtains in transmission but not in reception.As shown in Fig. 3, in transmission the wavelets outgoing from aperture9 travel along the divergent radial paths i2 and I3 and the waveletsreturned by the two reflectors to aperture 9 travel along the radialpaths I6 and I8. In reception, however, while the wavelets incoming to,and reflected by, the two reectors are propagated along radial paths tothe aperture, the wavelets incoming from the distant station to the tworeilectors travel along parallel paths of equal length. Accordingly, thecornu spiral does not'represent the received .energies and the dimensiona of. the smallrelector is' not: such as to cause cancellation .ofl thewavelets reflected by the two reiiectors. Moreover, the auxiliaryreector .is more or less shadowed by the :primary'yantennaand its`eitecton Vthe receiving operation is negligible.` J

Thus far, the operation of the three'. embodimenta Figs.. 1, 3 and. '2,3, and Llpat onlythe mean or' design frequency has been considered.'Referring now to Fig. 8 and assuming the auxiliary reflector l, Fig. ,1or 2l, Fig. 2, is omitted and the transmitter, frequency is changed orvaried, the intensity of the standing wave pattern at a .givenpoint intheI guide passes throughV maximum and minimum values as the focallength R, in wavelengthsof reflector lv or 20 passes successivelythrough quarter-wavelength values, that is, as Vthe Vround trip distance2R inV wavelengths rsuccessively assumes values Corresponding@ azmultpleof a half wavelength. The .greater the value of R, the closerfin-.timeor wavelength separation the successive maximum or minimum values areand for a large value of B., `such as R=7.75 wavelengths, the maximumvalues are very close together and anarrow fref quency band is obtained.Stated diierently, when the distance 2R is very large the difference inwavelengths between two waves producing, at the givenpoint inthe guide,adjacently suce cessive maxima is a small percentage of l thedis-f tance2R. whereas, iwhen 2B, is small, the aforesaid difference is a largepercentageof thedis'- tance*Y 2R, When the auxiliary lreflectorisutilized the mismatch effect, is` not only; cor; rected at the meanfrequency but is minimizedat substantially all frequenciesinthefband-,since' the thickness, c7,=7\/'7.,.is` a relatively smallfraction oflall wavelengthsin the. band.

Referring to Fig. 8, .the dotted curve 36 and the full line curve 3l'illustrate, respectively,A the measured'bandwidth characteristics, takenover the SL45-10 centimeter band, for the` system of, Fig. 4 without,and with, the auxiliary reector l0. As shown in thisgiigure, in ythecentral 9.5-9.85 centimeter portion of the band,the.curv`e 38 for theprior art system contains the highly undesirable maximum standing wavepeak 38 at 9.65 centimeters. The two minima 39 vrelatively closetogether and the two maxima 38 arehrela tively close together` The twousable portions or bandwidths, eachcentered on aminimumY point 39 andextendingk below ve decibels,a'refver'y` narrow. On the other hand, thecurve v3.1 v is relatively flat over theY wide band"9.5,9'.9,"and. inthe central region contains no -maximum-pointso that the system oftheinvention has-.a veryy wide operating bandwidth.

Referring to Figs. 9, l0 and 1l,`the system lof Figs. 9 and 10 isthe'same as that of Figsl`1and3. except that the auxiliary' reector 40comprises two strip reflectors 4l and 42 which are attached' to thereflector l in'place Iof the' single strip auxiliary reflector I0. vThestripv reflectors are spaced equally fromfthe axial plane 4 and extendparallel to the .vertex line 2 and focal line. 3of` reflector l.- Thesystem .of Figs".- 1'0 and 1lis. the same as that of Figs. 2.and` 3except 'that aring reflector .43 is used in placelof 'the disk 're--4flector 2l, the .ring reflector 43 and the para-` boloidal reflector 20being coaxially disposed. Asexplained below, thespacing a1 vand thespacingA a2, Piggabetween the axial man@ 4, Fig. 11, and thelinner edges44 and theouterv edgesf45, re` spectively, of the strip reflectors 4|and the thickness dm ofv each strip reflector, aresuchas to suppress thestanding lwave established in,` guidev 5 by the main reflector l,Similarly the spacing a1 and thespacing'azvbetwe-en the axis-221of-reiietor 2li and the inner edge 46 and the outerY edge 410i theringreector v43, and. the thick-:1

- ness dm of reflector 43are such as to cancel stand;

guide 5 by the paraboioidaL fao'aoio standing The longitudinaldimensioni.. of the reflector I in the system of Fig. 4 is very short ascompared to that of reector I in Figs. 1, l3; and in Fig. -4 the guide 5extends perpendicularto'fthe latus rectum of reflector I whereas Vinthe'embodiment of Figs. A`1, 3 it is parallel to the latus` rectum.Also, in the system of Fig. 4,`the` rei-lector l is equipped with aconductive top end lplate 23 and a conductive bottom end plate 24 which,togetheriwith the transverse ends of reflector I, form a rectangular orsubstantially linear antenna aperture. VThe secondary lantennacomprising'the cylindrical-parabolic reflector I and the end plates 23,24 constitute-a so-called ',pillbox, cheese-box or parallel plateantenna reectcr. A conventional pill-box 'antenna reflector isYillustrated on page 840 ofthe textbook Radio Engineers Handbook by F. E.Terman. In generaLrthe operation ofthe embodiment of Fig. 4 is the samekas described above for the cylindrical-parabolic system of Figs. l-and3, and a fan-beam is obtained. The plates 23. 24 function as guidemembers for the energy emaeA nating from the primary antenna aperture 9so thatthe illumination or energization of reflector I is relativelyintense and a highgain is secured. The parallel `plates also undesirablyincrease, at least to some extent, the feedback, or reflection fromreector l towards the primary antenna aperture 9. Hence, thelutilization of a strip reflector I0, for the purpose of cancellingthefeedback energy, isin a sense of more signicanoe in the case of thesystem of Fig. 4 than in the system of Figs. 1, 3. j

Referring to Figs. 5, 6 and 7 the method of determining the dimensions dand 2a of 'the strip and disk reflectors I0, 2I will now be explained.First of al1, the amplitude of vibrationv at a point P, Fig. 5. of thewaves received from a source S'via a reflector 25, the point P andsource S being at the respective 'distances Rp and Rs from the surface25, may be represented by the upper-half 25 of a cornu spiral 21,v Fig.7, as explained in the textbook Theoretical Physics by Georg Joos. pages376-378. Thus, rif the width or height of surface 25 were infinite, theamplitude at point'P may be representedby the vector 28, Fig. 7,extending from the origin 29 to the asymptotic point 30, that is,between the opposite terminals, of the half spiral 23. The lengthdimension of vector 28 represents the resultant or integrated amplitudeof the wavelets originating at-Sand impinging on the 'surface 25; andthe angle between the vector 28 and the XX axis represents theintegrated phase of these wavelets. If the width a of the surface 25 isfinite and has a dimension corresponding to an intermediate point on thespiral A, that is, corresponding to a selected value of the parameter Aofthe spiral 26, an amplitude is obtainedat the receiving point P whichmay be representedby the .vector 3| extending from the origin 12B to theintermediate point. If the surface 25 begins at an intermediate point,Fig. 7, corresponding' to the selected value of the parameter A andextends to innity, the amplitude at vpoint P maybe represented by thevector 32 extending from the intermediate point to the asymptotic point30.

.Applying the above theory ftothe er'nbodiments of Figs; 1, 2 and 4, thesource y'Sand point Picci"- respond to the aperture 9 so that'RP:R,"Fig.3, the focal length of reflector I, Figs. 1 andf4', or reflector 20,Fig. 2. `Also the parabolic curvatures of reflectors l and 20 are suchthat these reflectors reasonablyapproximate the flatl surface 25, Fig.5. It is apparent from the above that the surface 25, Fig..5, may bedivided into two parts 33 and 34,Fig. 6, which have somewhat criticalwidths and which produce two vectors 3l and 32 having equal lengths andtherefore equal amplitudes.-l Moreover, the thickness d or difference inthe lengths of the paths ex- ,A-vm or, since in Fig. 3 RpzRsr-R ,(2)

y 3 A a\/XR l(` where 't is sthe'mean. Yor design wavelength and, asalready indicated, A is a parameter of the Cornu spiral. YFor plane orflatV reflecting surfaces 33 andi-34, and for the condition 'vector' 3lequals vector 32, the Value of A, as scaled from the cornu spiral'26`isy Y .420.47 (5) hence,

For one embodiment constructed in accordance with Figs. 1 and `3 andsuccessfully tested.r

It follows from the above that the critical or optimum dimensiony 2a forthe width ofthe fiat stripfreilector ID, Figs. 1, 3 and 4, used vin theYembodiments comprising a cylindrical parabolic reector having a focallength of 30.5r inches. is 5.24" or 1.33 wavelengths. For the comparableparaboloidal system, Figs. 2 and 3, the :optimum diameter 2a of the fiatdiskv 'reiiector V2I is 5.24 inches. If the reflecting' surface of the:strip or disk reflector is parabolic, instead of at, the parameter Alis slightly larger than .0.47 as, for example, 0.6 and the dimension2a, is greater than 5.24 inches. y

The thickness d of the strip reflector I0, and of theY disk reflector 2I, may be determined from Fig. 7 by scalingfthe phase angles V62.1 and632, respectively, *or vectors 3|` and'32 and rotating vector `3lthroughfan vangle 0 to the position slightly illustrated by vector 35 atwhich the sum of the phase,V angle 035 and the phase angle 032 is 180degrees. The thickness d in wavelengths of each of reflectors I and 2lVcorresponds to one-half of vthe phase angle .0. As scaled from Fig. 7,a equals about 103 degrees. In the tested embodiment o f Figs. 1 and 3,the thickness ==0-565 :1.43 cm. (l1) was therefore utilized. The valueM7 is not extremely critical since successful results are obtainablewhen d has any walue included in the range of M5 to M9.

The transmitting and receiving directive patterns for the system ofFigs. l, 3, or for the system of Figs. 2 and 3, are the same, inaccordance with the well-known reciprocity theorem. In the transmittingand receiving patterns, the top or peak of the major lobe is slightlydepressed by reason of the front position of the primary antennaelement. The primary element shadows the vertex portion of the mainreflector and the attached auxiliary reflector, and the auxiliaryreflector does not, therefore, appreciably affect either pattern. Themutual wave cancellation, however, obtained in transmission at theprimary antenna and described above, differs considerably from thatobtained in reception. The cancellation of the wavelets reflected by themain and auxiliary reilectors into the primary antenna or aperture 9,Fig. 3, is predicated on the assumption or condition that the outgoingwavelets travel along divergent radial paths of different length fromthe source S. Fig. 6, or aperture 9, and the components of the reflectedwaves return to the aperture?! travel along convergent radial paths ofdifferent lengths. yThe cornu spiral, Fig. 7, and the associatedequations (l) and (6) from which the value of dimension a oftheauxiliary reflector is obtained, are based on the assumption orcondition just described. This condition obtains in transmission but notin reception. As shown in Fig. 3, in transmission the Wavelets outgoingvfrom aperture 9 travelA along the divergent radial paths l2 and i3 andthe wavelets returned by the two reflectors to aperture 9 travel alongthe radial paths I6 and i8. In reception, however, while the waveletsincoming to, and rellected by, the two reflectors are propagated alongradial paths to the aperture, the wavelets incoming from the distantstation to the two reflectors travel along parallel paths. of equallength. Accordingly, the cornu spiral does not represent the receivedenergies and the dimension a of the small reflector is not such as tocausecancellation of the wavelets reected by thetwo reflectors.Moreover, the auxiliary reflector is more or less shadowed by theprimary antenna and its effect on the receiving operation isYnegligible.

Thus far, theoperation of the three embodiments, Figs. l, 3 and 2, 3,and 4 at only the mean or design frequency has been considered.'

through maximum and minimum values as thev focal length R, inwavelengths, of reflector l or 20 passes successivelyjthroughouarterwavelength values, that is,as,the roundtrip distance 2R.in wavelengths successively assumes,- Values correspondingto a multipleof a half wavelength. The Igreater the valueof R, the closer in-` timeor wavelength separation Vthe successive mum or minimum values are; andfor a large Nalue of R, such as R=7.75 wavelengths, the maximum .valuesare very close togetherand a narrowjfre quency .band is obtained. Stateddifferently, whenthe distance 2R is very large the difference in--wavelengths between two waves producing, at the given point in theguide, adjacently successive maxima is a small percentage of thedistance 2R. Whereas, .Iwhen 2R is small, the-,aforesaid difference is alargepercentage of the distance 2R. When the auxiliary reflector isutilized the mismatch effect is not only corrected at the meanfrequencybut is minimized at substantially all frequencies in the band,since the thickness, d=l\/7, isa-relatively small tfracf tion of allwavelengths in the band. i

Referring toV Fig. 8, the dotted curve 33 and the full line curve 31illustrate, respectively,V the measured bandwidth characteristics, takenover the 9.45-10 centimeter band, for the system of, Fig. 4 without, andwith, thel auxiliary reflector l0. As shown in this figure, inV thecentral 9.5-9.85 centimeter portion of the band,`,the curve 35 for theprior art system contains the highly undesirable maximum standing wavepeak 38 at 9.65 centimeters. The two minima 39 relatively close togetherand the two maxima 38 are rela. tively close together. The two usableportionsor bandwidths, each centered on a minimum point 39 and extendingbelow five decibels, are very narrow. On the other hand, thecurveffilfisx relatively flat over the wide band 9.5-9.9, and in thecentral region contains'no maximumpoint,A so that the system of theinvention has avery.l wide operating bandwidth. j Referring to Figs. 9,10 and 11, the system of; Figs. 9 and 10 is the same'as that of Figs'fland y3'. except that the auxiliary reflector 40 comprises two stripreflectors Hand 42 which are attachedv to the reector l inplace of thesingle strip auxiliary reflector lll." The strip reflectors Aare spacedequally'from the axial plane4 and extend parallel to the vertex line 2and focal lineI 3 of reflector l. The system of Figs. 10 and 11 is thesame as that of Figs. 2 and 3 except that a. ring reflector 43 is usedin place of the disk reguide 5'by the main reflector l.v Similarly the.k

spacing a1 and the spacing c2 between theaxis 22,

of reflector 20 and the inner edge 46 and the outer.

edge 47 of the ring reflector 43, and the thickness dm of reflector 43,are such as to cancel stand. ing waves produced in guide 5 by theparaboloidal` reflector 20. l

Referring to Fig.V 12, the manner of determining the dimensions ai anda2 will now be explained..

First, the three component vectors 41, .48 and 49,

having equal lengths are disposed on the cornu' spiral 21 so as toobtain two particular values Ai.- and Az of the parameter A. Theparticular Value' Ai corresponds to ai and is at the one-third point,

and the particular value A2 corresponds to azand' `41 and 54.

and Y Now, from equations (4),' (7) and (8) we have Hence, the widths ofreflectors 4l and 42, and the radial width of the ring reector 43, areeach a2-a1`=2=0.510l\ (16) The vector 48 corresponds to the waveletsreflected by the double auxiliary reflector 40, Fig. 9, and the vectors41 and 49 correspond respectively to the wavelets reflected by thevertex or inner portion 50, Fig. 9, and the 4outer portion 5| of thecylindrical parabolic reflector l. Also, vector 48 corresponds to thereflection from the ring reflector 43, Fig. l0, and the vectors 41 and49 correspond respectively to the reflections from the vertex or innerportion 52 and the outer portion 53 of reilector 20. Vector 54 isidentical with vector 49 and vector 55 represents the resultant ofvectors By selecting a proper thickness dm for each of the stripreflectors 4l, 42, or for the ring reflector 43, the vector 43 may berotated clockwise through the angle 6m to the position illustrated byVector 56, at which the sum of the phase angle s of vector 5B and thephase angle 05s of vector 55 equals 180 degrees. Thev physical thicknessdm of the reflectors 40 and 43, in wavelengths, is selected so as tocorrespond to one-half the scaled value of the angle 0m. Since, from thecurve, 0m equals about 144 degrees, we have =72 degrees (17) Cilly It isthus apparent that, in the embodiment of Figs. l and 4, each comprisinga cylindrical-parabolic reector, rthe mismatch effect may be correctedby utilizing, in place of the single-strip reflector l0, thedouble-strip reflector 40, Fig. 9. In particular, the single-stripauxiliary reflector used in the tested embodiment described above andhaving a thickness of one seventh of the mean Wavelength may be replacedby a dual-strip auxiliary reflector comprising two metallic stripreflectors 4l and 42, one on each side of the axial plane 4 and eachhaving a thickness of a fifth of the mean wavelength. The reflectors 4Iand 42 have their inner edges 44 at 0.554 wavelength from the axialplane 4 and each has a width of 0.510 wavelength. Similarly, in thecomparable paraboloidal embodiment, Fig. 2, the mismatch may beeliminated by employing, in place of the disk reflector 2l, a coaxialring reflector 43 having a thickness in the order of M5, the inner andouter radii of the, disk reflector being 0.554 and 1.064 wavelengths,respectively.

In the foregoing description it has been assumed that the main reflectorhas a very slight curvature corresponding to the curvature of a parabolahaving a very large focal length, and that the size or opening of thereflector is fairly small, com- -fparabie to the natively nat portion fthe' aferrasaid parabola in the region of its vertex'or axis, wherebythe main reflector may be regarded as flat, as shown in Fig. 6. Ifthefocal length is verysmall the main reflector may have a pronouncedcurvature and, preferably but not necessarily, ais'lightj correctionforthe curvature may -be1made. Again. in thecase ofA reflectors havingshort focal lengths, the intensity lof the illumination of theperipheral portiorsz'of thejmain Vreflector, and hence the intensityofthe energies fed antenna may be made.. These corrections are relativelysmall and may easily be made by one skilled in the art.

Although the invention has been explained in connection with specificembodiments it is to be understood that it is not to be limited to theembodiments described inasmuch as other apparatus and arrangements maybe used in successfully practicing the invention. To illustrate,satisfactory results may be obtained by positioning the auxiliaryreflector at a critical distance in back of the main reflector andproviding a properly dimensioned and properly located aperture or windowin the main reflector.

What is claimed is:

1. In combination, a primary antenna element for transmitting andreceiving radio energy of a given wavelength, a large main reflector, aplurality of spaced small reflectors positioned at equal distances fromvsaid element and between said element and said main reflector, thespacing between said small reilectors and the reflective area of eachsmall reflector being dependent upon said wavelength.

2. A combination in accordance with claim l, the distance between saidelement and said main reilector being approximately 0.20 wavelengthgreater than the distance between said element and said plurality ofsmall reflectors.

3. In combination, a large cylindrical-parabolic reflector having anaxial plane and a vertex line,

'a dielectric guide having at one end an antenna aperture, said apertureextending perpendicular to and having its mid-point included in saidaxial plane, a plurality of parallel strip reflectors attached to thefront of said large reflector, said strip reflectors being positioned onopposite sides of and at equal distances from said vertex line, thespacings between said aperture and said strip reflectors being equaland'diiferent from the spacing between said aperture and said largereflector, and a transmitter connected to the other end of saiddielectric guide.

4. In combination, a large substantially flat reflector having aparabolic contour and a focal length R, a primary antenna element at thefocus of said reflector for radiating waves having a wavelength A and acircular wave front in the plane of said contour, a first smallreflecting member included between said large reflector and saidelement, said small member being perpendicular to and spaced from theaxis of the large reflector, the distance between said axis and the edgeof said small member nearer to said axis being equal to and theseparation between said axis and the edge vof said small member` fartherfrom said aXis being and a seond small reecting member includedREFERENCES CITED i' f 1 I The following references are of record in theY f 0.75\/T1- l file of this patent: v

5 UNITED STATES PATENTS `between said large reflector and-said element,said Number Name Date secondsmall--inemberbeing positioned on the2,118,419 Scharlau May 24, 1938 o the Vside of and spaced -from saidaxis, the -dis- 2,429,601 BiSkebOfn et a1,- Oct. 28, 1947 tanee betweensaid axisandizhe edge of said secl0 FOREIGN PATENTS ondmernber nearestsaid axisbemg equal to the rs mentioned distance and the separation be-Number Country Date tweenjsadaxis andthe edge of said second mem-4101336 Great Britain May 171 1934 ber-fartherA from said axis beingequal to said first

